modify DEVELOPER_NOTES.md to include user guide generation details

Author: Jonathanlyj

DEVELOPER_NOTES.md

@@ -2,12 +2,14 @@
 ---
 
 ### Note on configuring Read the Docs tool for user guide generation
+ * Read the Docs settings
+   * User guide is automatically generated on main branch changes and opened PR requests. Modify this in Read the Docs dashboard if needed
  * Doc files
    * `.readthedocs.yaml` -- The script that controls commands that run before installation of pnetcdf-python (under `pre-install:`) and installation (under `python:`)
    * `/docs/requirements.txt` -- Python dependencies required for doc generation, including `sphinx`
  * Important environment variables
-   * User guide generation requires enviroment variables set by Read the Docs dashboard (`Admin` -> `Environment Variables`). Delete and add a new variable to modify
-   * Current environment variables set:
+   * User guide generation requires enviroment variables set by Read the Docs dashboard (`Admin` -> `Environment Variables`). Delete and add a new variable to modify (remember to select `Expose this environment variable in PR builds` if PR auto-build is enabled)
+   * Current environment variables set (only effective solution found to set env variable at installation):
      * `CC`: mpicc.mpich
      * `PNETCDF_DIR`: /home/docs/checkouts/readthedocs.org/user_builds/pnetcdf-python/checkouts/latest/_readthedocs/PnetCDF
      * `PNETCDF_VER`: 1.12.3